Pseudo-Jahn-Teller origin of phonon anomalies in IV-VI compounds

Phonon anomalies in the high-symmetry phase of IV-VI semiconductors are analyzed in the framework of the vibronic theory of ferroelectricity. It is shown that, in the vicinity of the ferroelectric phase transition, the specific features of the lattice dynamics are determined by pseudo-Jahn-Teller interactions, and the interatomic potential depends on the symmetry of the lattice distortions. The above-mentioned approach is used for calculating the phonon spectra of PbTe and PbS.